Hysteresis drivers in the form of hysteresis clutches or hysteresis brakes have long been known in many forms. The advantage of such clutches or brakes consists essentially in their ability to transmit torque without contact, across an air gap. The way these devices work relies on the magnetic force action of mutually attracting poles in synchronous operation or a continual magnetic reversal of a permanently magnetic hysteresis material moved past these poles in slipping operation. For example electromagnetically energizable hysteresis clutches are known from DE 39 05 216 A1 and DE 199 17 667 A1, whose transmitted torque can be adjusted as a function of the current flowing through an energizing coil.
In addition, from DE 37 32 766 A1 a permanent-magnet-energized hysteresis clutch is known, in which the torque to be transmitted can be changed by manually varying the insertion depth of an annular hysteresis element into an air gap formed between two pole rings of the permanent magnet.
Furthermore, from DE 2 261 708 A it is known to operate a hysteresis clutch of an auxiliary aggregate drive of a motor vehicle in such manner that it is activated or deactivated as a function of the temperature of the coolant liquid or oil of an internal combustion engine. This hysteresis clutch comprises electromagnets that can be switched on, i.e. a plurality of electromagnetic fields, with poles complementary to one another.
Moreover, from DE 197 46 359 C2 and DE 100 18 721 A1 adjustable coolant pumps for motor vehicles with hysteresis clutches are known. The first of these documents describing a permanent-magnet-energized hysteresis clutch, one clutch half of which can be displaced axially by means of an electrically driven adjustor unit so that the gap width of the air gap between the two halves of the clutch, and consequently the torque to be transmitted, can be varied as a function of the operating condition of the combustion engine. On the other hand DE 100 18 721 A1 concerns an electromagnetically energized hysteresis clutch by which the torque to be transmitted can be regulated or adjusted as a function of the size of the current flowing through the coil of an electromagnet.
These known hysteresis clutches are similar in that they all use electric or electro-mechanical regulation of the torque transfer, which is associated with the disadvantage that if the current supply to such hysteresis clutches should fail, they can no longer carry out their intended purpose.
Against that background the purpose of the present invention is to propose a hysteresis driver, such as a hysteresis clutch or a hysteresis brake, whose adjustment or regulation is improved, in that on the one hand it enables continuously variable torque adjustment and on the other hand it can still transmit torque even if the current supply has failed. Such a hysteresis driver should for example also be suitable for use as an auxiliary drive of a motor vehicle engine or an auxiliary drive output.